GOBLET CELL ALKALINE-PHOSPHATASE IN SILKWORM MIDGUT EPITHELIUM - ITS ENTITY AND ROLE AS AN ATPASE

The characteristics of two alkaline phosphatases (ALP) in the midgut epithelium of the silkworm Bombyx mori (Insecta: Bombycidae) have been investigated further. Distributions of the soluble ALP and membrane-bound ALP were determined quantitatively by rocket immunoelectrophoresis with monospecific antibody against the purified soluble ALP and membrane-bound ALP, respectively. More than 70% of soluble ALP was distributed in the posterior midgut, and the rest of it in the anterior and middle midguts. The membrane-bound ALP existed at similar levels in all regions of midgut except the most anterior portion. Enzyme-histochemistry of ALPs could be detected at the apical cell surface of goblet and columnar cells as shown in the previous paper on the immunohistochemistry of ALPs (Azuma, M., and M. Eguchi (1989) J. Exp. Zool. 251:108-112). Furthermore, it was shown that the soluble ALP (goblet cell ALP) required Mg2+ to express its maximum activity, but the membrane-bound ALP (columnar cell ALP) was insensitive to Mg2+. By using ATP as the substrate, the purified soluble ALP possessed ATPase action with Mg2+- and/or HCO3--dependent manner. From these results, it is concluded that the soluble ALP functions as one of the ATPase at the goblet cell surface, differing from the K+-ATPase. This ATPase may be a possible component in generating the alkalinization of midgut lumen characteristic of lepidopteran insects.